The physiologic activities of the recirculating pool of thymus-derived (T) lymphocytes are studied in murine animals in an effort to understand further the function of these cells in immune responses. Experiments are performed on the relationship between lymphocyte surface structures and the distribution of these cells in the body. Myxo and paramyxoviruses which differ in their binding to T cells are used to study surface receptors which determine their route of migration through the body. The differentiation of lymphocyte subpopulations by membrane receptors for these viruses is also being investigated.